In a Pirani vacuum gauge, a current is flowed through a filament to heat the filament. An amount of heat absorbed by the surrounding gas from the filament depends on the gas pressure, and therefore the gas pressure is measured by utilizing this dependency. The filament may be in the form of a coil, for example, as shown in the patent literature 1.
Patent Document 1: JP7-120339A
FIG. 7 shows a schematic construction of the prior art Pirani vacuum gauge. The filament 1 is included in a cylindrical body 2, which is open at an upper end and is closed air-tight at a lower end with an insulating member 4. The opening is communicating with the space s to be measured for pressure and the interior of the cylindrical body 2 also is communicating with the space s, which is the interior of a vacuum tank 11. The filament 1 contacts with the space s to be measured for pressure. The cylindrical body 2 functions as the division between the space s and the atmosphere outside of the vacuum tank 11.
The lower end of the filament 1 is connected to the conductive terminal 5b, which is inserted through the insulating member 4 into the cylindrical body 2. The top end of the filament 1 is connected to a top end of a conductive filament support 6 and the lower end of the conductive filament support 6 is connected to the conductive terminal 5a, which is inserted through the insulating member 4 into the cylindrical body 2. Accordingly, the filament 1 is electrically connected to the conductive terminals 5a and 5b, which are connected to a not-shown control circuit under the atmosphere outside of the cylindrical body 2. Electric power is supplied to the filament 1 through the conductive terminals 5a and 5b. 
The filament 1 is assembled into a part of a not-shown bridge circuit and resistance change with temperature change of the filament 1 is detected by the bridge circuit, in the marketed Pirani vacuum gauge. There are operation modes of the constant current or voltage type and constant temperature type. In the Pirani vacuum gauge of the constant voltage or current type, a constant current or voltage is supplied to the bridge circuit. The resistance change of the filament 1 with temperature change thereof due to the gas pressure change is detected as the unbalanced voltage of the bridge circuit. In the Pirani vacuum gauge of the constant temperature type, with the detection of the unbalanced voltage, a current is so fed back to the bridge circuit that the resistance or temperature of the filament 1 is maintained at constant and the balance of the bridge circuit is maintained. The electric power is so controlled automatically that the amount of the heat absorbed by the gas is compensated to maintain the temperature of the filament 1 at constant. Accordingly, the gas pressure is detected with the supplied electric power. The usual measurement accuracy is about ±30% under the pressure range (less than 3×10 Pa) usually measured by the Pirani vacuum gauge.